A communication system is a facility that enables communication between two or more entities such as user equipment and/or other nodes associated with the system. The communication may comprise, for example, communication of voice, data, multimedia and so on. Communication is shifting to communication networks such as IP (Internet Protocol) and/or other packet switched data networks. A communication system typically operates in accordance with a given standard or specification which sets out what the various elements of the system are permitted to do and how that should be achieved. For example, the standard or specification may define if the user, or more precisely, the user equipment or terminal is provided with a circuit switched service and/or a packet switched service. Communication protocols and/or parameters which shall be used for the connection may also be defined. In other words, a specific set of “rules” on which the communication can be based needs to be defined to enable communication by means of the system. The communication between the user equipment and the elements of the communication network can be based on an appropriate communication protocol, such as the SIP (Session initiated protocol), RTSP (real time streaming protocol) or some other protocol.
RTSP is a standard protocol for controlling multimedia streaming on Internet. The RTSP-protocol however has some disadvantages. In particular, RTSP does not provide for media negotiation and the client and server cannot dynamically select best possible media formats. Media negotiation was handled at application level, for instance, several RTSP URL to a particular media resource could be given, each with particular media parameters. The RTSP DESCRIBE method can be used to retrieve stream parameters from the RTSP streaming server. No negotiation is done and the streaming is possible only if the client supports the media and codecs listed in the response to DESCRIBE.
SIP (Session Invitation Protocol, RFC 3261) is a popular protocol used to setup multimedia sessions. Its applications include, for instance, Internet telephony and conferencing. SIP provides means for publishing endpoint capabilities, negotiating session contents (which media are used, which codecs are used, etc.) and updating sessions. It also includes means for subscribing to events and delivering event documents whenever the event status has been changed. It can handle negotiations for communications sessions dynamically. However, the SIP protocol does not support multimedia streaming.